Solid state lighting remains an elusive goal, because the broad spectrum quality white light is not provided. Research into solid state lighting has been conducted since the introduction of the first commercial light emitting diodes (LED) in the 1960s. Initial systems lacked a blue component, and blue emitting LEDs were developed much later. Since the introduction of the blue LED, there have been many proposed systems to produce white light from LED sources.
Example systems are the blue LED-pumped systems. These systems do not use a blue phosphor component. The blue component of the white light is thus provided directly from the pumping LED. A recent advancement in such systems is provided by Scianna et al, U.S. Pat. No. 8,143,079. That patent describes use of a white light emission device that has a cascade configuration of luminescent silicon nanoparticle films to convert the output of a UV/blue light LED into white light output. Red, green and blue films are stacked on the UV/blue light LED. These films allow the blue light of the LED to pass through, but absorb the UV light. The absorbed UV light produces respective red, green and blue fluorescence from the cascaded nanoparticle films. The device produces wide spectrum white light.
However, reliance on the blue LED pumping source presents a significant hurdle to achieving a high correlated color temperature (CCT) and color rendering index (CRI) at the same time. These are measures that help compare the quality of a white light source to natural light.
Others have proposed using high-power UV LEDs to drive white light generation. UV radiation is potentially harmful and its transmission must be limited. High power UV LEDs have to be used in a configuration that captures and converts the UV radiation. This conversion requires an efficient wide band red converter. Few good efficient red phosphors, whether sulfide-, nitride-, or oxide based have been known. Typical spectra from known converters are dominated by sharp line spectra with branching ratios that depend on the UV wavelength, which is not ideal for color mixing. The red phosphor yttrium oxide-sulfide activated with europium (Y2O2S:Eu), for example, has been investigated in UV-based lighting. Co-doped phosphate materials were recently synthesized for near UV pumping, which provided a peak wavelength of 610 nm. See, Cho et al, “Study of UV excited white light-emitting diodes for optimization of luminous efficiency and color rendering index,” Phys. Status Solidi (RRL) 3, 34 (2009).
Another approach for wavelength conversion on a UV-LED based source has been (CdSe)ZnSe quantum dots to produce a hybrid red emitting LED. See, Song et al., “Red light emitting solid state hybrid quantum dot-near-UV GaN LED devices,” Nanotechnology 18 255202 (2007). The (CdSe)ZnSe quantum dots were used as red phosphors and a GaN UV-LED provided excitation. This device did not provide white light emission, however, instead only providing red emissions.
Present red phosphor converters provide spectra dominated by sharp lines and suffer from availability and stability issues which are not ideal for color mixing in display or solid state lighting applications.